1. Field of the Invention
The present invention relates in general to evaporation emission controls (EEC) of motor vehicles powered by internal combustion engines, which prevent the escape of gasoline vapors from the fuel tank and fuel supply system, whether or not the engine is running More specifically, the present invention relates to a gasoline vapor purging system for use in an internal combustion engine of a gasoline direct injection type.
2. Description of the Prior Art
For preventing the escape of gasoline vapors from the fuel supply section such as a fuel tank or the like, most vehicles built during these days use an activated charcoal canister to trap the vapors when the engine is shut off. Upon restarting, a flow of filtered air through the canister purges the vapors from the charcoal. The mixture goes through a vapor purge pipe feeding into an intake section of the engine and it is burnt in the engine.
In a gasoline vapor purging system disclosed by Japanese Patent First Provisional Publication 7-42588, a purge control valve is used for controlling the flow off vapors in the vapor purge pipe in accordance with an operation condition of the engine. That is, in the conventional vapor purging system, a target vapor purging rate (viz., target open degree of the purge control valve) is determined in accordance with the flow rate of air (viz., intake air) led into the engine.
However, hitherto, the vapor surging system has been given little thought in internal combustion engines of a type which can switch between a so-called "stoichiometric combustion mode" wherein the engine operates on a mixture of stoichiometric air/fuel ratio and a so-called "lean combustion mode" wherein the engine operates on a mixture of higher (or lean) air/fuel ratio. That is, to allow the engine to operate in the lean combustion rode, it is necessary to increase the flow rate of the intake air by increasing the open degree of a throttle valve. Thus, if the target vapor purging rate is determined in accordance with the flow rate of the intakes air like in case of the Publication 7-42588, the lean combustion mode is forced to induce excessive vapor purging, which tends to lower combustibility of the mixture in combustion chambers.
In internal combustion engines operating on gasoline, there is a so-called "gasoline direct injection type" wherein fuel (viz., gasoline) is directly injected into combustion chambers. The engines of such type disclosed by Japanese Patent First Provisional Publications 62-191622 and 2-169834 have two combustion modes of early and late injection of fuel. That is, injection early in the intake stroke produces a homogeneous mixture for high-load conditions, while the late injection delays the event until near the end of the compression stroke to create a stratified mixture for low-load conditions. The stratified mixture generally consists of a first layer of richer mixture which is easily ignited by an ignition plug and a second layer of leaner mixture which is ignited by the burning of the first layer of mixture. Combustion of the stratified mixture realizes ultra-lean combustion and thus brings about improvement in fuel economy due to reduction in pumping loss.
However, as is known, combustion of stratified mixture is very sensitive to a change in air/fuel ratio and thus easily affected by the above-mentioned vapor purging and the exhaust gas recirculation (EGR). That is, if the stratified mixture becomes to have an outer layer whose air/fuel ratio is too high (viz., too lean) due to affection by the vapor purging and/or the exhaust gas recirculation (EGR), appropriate combustion of the outer layer is not obtained, which increases a possibility of remaining unburned substances in the combustion chambers. While, if the stratified mixture has a too low air/fuel ratio (viz., too rich mixture) in total, appropriate combustion of the mixture is not obtained also, which increases a possibility of exhausting much smoke and thus increasing fuel consumption,
For solving the above-mentioned drawbacks, a measure may be thought out wherein during combustion of the stratified mixture, the vapor purging is inhibited or at least turned Down. However, in this case, the performance of the gasoline Vapor purging system per se is sacrificed. That is, fuel (viz., gasoline vapors) trapped by the charcoal canister can not be completely purged from the charcoal canister under operator of the engine, so that it tends occur that when the engine is stopped, the canister exhibits its saturation condition in adsorbing vapors. In this case, gasoline vapor generated thereafter can not be trapped is by the canister any longer, which permits escape of the vapor into the atmosphere.